01 - Preheat your oven to 350°F. Line a baking sheet with parchment paper.
02 - Slice each croissant horizontally without cutting all the way through, creating a hinge.
03 - Spread a thin layer of Dijon mustard inside each croissant, if using.
04 - Layer one slice of ham and one slice of cheese inside each croissant.
05 - Close the croissants gently and brush the tops with melted butter. Arrange on the prepared baking sheet.
06 - Bake for 12–15 minutes, or until the cheese is melted and the croissants are golden and crisp.
07 - Serve warm.